The present invention relates to a printing method for transferring a printing paste retained on a stencil (plate) onto an object on which a print is to be formed and printing apparatus for implementing the printing method.
Conventionally, according to a planographic stencil (screen) type printing for printing, for example, solder paste on lands of a printed circuit board, as shown in FIG. 18A and FIG. 18B, a screen mask (metal mask) 1 having through holes 1a arranged in a specified pattern in correspondence with lands 5 of the printed circuit board 4 is placed in a specified position on the board 4 while being brought in contact with the board. Next, as shown in FIG. 18C, FIG. 19A and FIG. 19B, solder paste 2 is supplied to one end of the screen mask 1, and this solder paste 2 is moved by a squeegee 3 from the one end of the screen mask 1 in a specified direction, thereby filling the solder paste 2 into each through hole 1a of the screen mask 1. Next, as shown in FIG. 18D, the screen mask 1 is separated from the board 4 so as to move the solder paste 2 inside the through holes 1a of the screen mask 1 onto the lands 5 of the board 4, thereby forming solder paste layers 2a on the lands 5 of the board 4 as shown in FIG. 18E.
However, according to the above structure, as shown in FIG. 19C, part of the solder paste 2 is left inside the through hole 1a of the screen mask 1 while adhering to the inner wall of the through hole due to the viscosity of the solder paste itself when the screen mask 1 is separated from the board 4, and this disadvantageously causes a phenomenon that the solder paste continuously extends across the left solder paste 2 and the solder paste 2 placed on the land 5 of the board 4. Consequently, as the screen mask 1 moves away from the board 4, the relative deformation (shear rate gradient) of the continuously extending solder paste increases to be pulled and broken at an arbitrary portion between the screen mask 1 and the board 4. Part of the solder paste that has been pulled and broken adheres to a portion other than the land 5 on the board 4 as shown in FIG. 19D and adheres to the peripheral portion of the through hole 1a on the rear surface of the screen mask 1 on the board side. This has disadvantageously caused a printing blur in the next printing stage, the occurrence of a bridge defined by the inadvertent adhesion of the solder paste to the adjacent solder paste layer 2a on the board 4, and the insufficient formation of a solder paste layer on the board due to the adhesion of the solder paste to the screen mask.
Accordingly, the object of the present invention is to solve the aforementioned issues and provide a printing method and printing apparatus capable of accurately pulling and breaking the printing paste between the stencil on which the printing paste is retained and the board while causing no bridging, causing no printing blur attributed to the printing paste left on the stencil side and causing no shortage of supply of the printing paste onto the board.
In order to achieve the above object, the present invention is constructed so that the temperature of the portion which belongs to the stencil and on which the printing paste of the stencil is retained is increased so as to reduce the viscosity of the printing paste that adheres to the printing paste retaining portion and allow the printing paste to be easily separated from the retaining portion, thereby making the printing paste easy to be printed on the object on which a print is to be formed.
According to a first aspect of the present invention, there is provided a printing method comprising:
retaining on a stencil a printing paste having a characteristic that a viscosity reduces as temperature increases;
increasing a temperature of a portion which belongs to the stencil and on which the printing paste is retained so as to reduce the viscosity of the printing paste to be brought in contact with the portion, thereby making the printing paste easy to separate from the stencil; and
separating the printing paste retained on the stencil from the stencil so as to print the printing paste on an object on which a print is to be formed.
According to a second aspect of the present invention, there is provided a printing method based on the first aspect, wherein the portion which belongs to the stencil and on which the printing paste is retained is heated by electromagnetic induction heating to increase the temperature of the portion.
According to a third aspect of the present invention, there is provided a printing method based on the second aspect, wherein the stencil has an opening portion to be arranged in a specified pattern for retaining the printing paste, and the stencil and the object are relatively separated apart after the stencil comes in contact with the object, thereby printing the printing paste inside the opening portion onto the object.
According to a fourth aspect of the present invention, there is provided a printing method based on the third aspect, wherein an electromagnetic induction heating unit for performing the electromagnetic induction heating performs the electromagnetic induction heating of the stencil in a noncontact manner.
According to a fifth aspect of the present invention, there is provided a printing method based on the fourth aspect, wherein an interval between the electromagnetic induction heating unit and the stencil is constructed so as to have a dimension such that a specified induction current flows through the stencil by the electromagnetic induction heating unit.
According to a sixth aspect of the present invention, there is provided a printing method based on the third aspect, wherein an electromagnetic induction heating unit for performing the electromagnetic induction heating performs the electromagnetic induction heating of the stencil in a contact manner.
According to a seventh aspect of the present invention, there is provided a printing method based on any one of the third through sixth aspects, wherein the electromagnetic induction heating is performed after retention of the printing paste on the opening portion of the stencil is finished.
According to an eighth aspect of the present invention, there is provided a printing method based on any one of the third through seventh aspects, wherein the opening portion is a through hole, the stencil is a screen mask, and the printing paste is filled into the through hole by moving a squeegee.
According to a ninth aspect of the present invention, there is provided a printing method based on any one of the third through eighth aspects, wherein a print state is detected after the printing paste is printed on the object, and an electromagnetic induction heating condition of the stencil or a condition of separation of the stencil from the object is controlled on the basis of a result of detection.
According to a tenth aspect of the present invention, there is provided a printing method based on any one of the third through ninth aspects, wherein the print material has a temperature gradient such that the portion put in contact with the portion retained by the stencil has a high temperature and the temperature gradually reduces departing from the portion in the electromagnetic induction heating.
According to an eleventh aspect of the present invention, there is provided a printing method based on any one of the third through tenth aspects, wherein an induction current for generating the electromagnetic induction heat flows in the lengthwise direction of the opening portion of the stencil.
According to a twelfth aspect of the present invention, there is provided a printing apparatus comprising:
a heating unit for increasing a temperature of a portion which belongs to a stencil for retaining a printing paste having a characteristic that a viscosity reduces as temperature increases and on which the printing paste is retained so as to reduce the viscosity of the printing paste to be brought in contact with the portion, thereby making the printing paste easy to separate from the stencil; and
a printing paste separation unit for separating the printing paste retained on the stencil from the stencil so as to print the printing paste on an object on which a print is to be formed.
According to a thirteenth aspect of the present invention, there is provided a printing apparatus based on the twelfth aspect, further comprising a stencil for retaining the printing paste having a characteristic that its viscosity reduces as its temperature increases.
According to a fourteenth aspect of the present invention, there is provided a printing apparatus based on the twelfth or thirteenth aspect, further comprising an electromagnetic induction heating unit for heating by electromagnetic induction heating the portion which belongs to the stencil and on which the printing paste is retained, thereby increasing the temperature of the portion.
According to a fifteenth aspect of the present invention, there is provided a printing apparatus based on the twelfth or thirteenth aspect, wherein the stencil has an opening portion to be arranged in a specified pattern for retaining the printing paste, and the separation unit separates the stencil relatively from the object after the stencil comes in contact with the object, thereby printing the printing paste inside the opening portion onto the object.
According to a sixteenth aspect of the present invention, there is provided a printing apparatus based on the fifteenth aspect, wherein the electromagnetic induction heating unit for performing the electromagnetic induction heating performs the electromagnetic induction heating of the stencil in a noncontact manner.
According to a seventeenth aspect of the present invention, there is provided a printing apparatus based on the sixteenth aspect, wherein an interval between the electromagnetic induction heating unit and the stencil is constructed to have a dimension such that a specified induction current flows through the stencil by the electromagnetic induction heating unit.
According to an eighteenth aspect of the present invention, there is provided a printing apparatus based on the fifteenth aspect, wherein the electromagnetic induction heating unit for performing the electromagnetic induction heating performs the electromagnetic induction heating of the stencil in a contact manner.
According to a nineteenth aspect of the present invention, there is provided a printing apparatus based on any one of the fifteenth through eighteenth aspects, wherein the electromagnetic induction heating is performed after retention of the printing paste on the opening portion of the stencil is finished.
According to a twentieth aspect of the present invention, there is provided a printing apparatus based on any one of the fifteenth through nineteenth aspects, wherein the opening portion is a through hole, the stencil is a screen mask, and the printing paste is filled into the through hole by moving a squeegee.
According to a twenty-first aspect of the present invention, there is provided a printing apparatus based on any one of the fifteenth through twentieth aspects, further comprising a control section for detecting a print state after the printing paste is printed on the object and controls an electromagnetic induction heating condition of the stencil or a condition of separation of the stencil from the object on the basis of a result of detection.
According to a twenty-second aspect of the present invention, there is provided a printing apparatus based on any one of the fifteenth through twenty-first aspects, wherein the print material has a temperature gradient such that the portion put in contact with the portion retained by the stencil has a high temperature and the temperature gradually reduces departing from the portion in the electromagnetic induction heating.
According to a twenty-third aspect of the present invention, there is provided a printing apparatus based on any one of the fifteenth through twenty-second aspects, wherein an induction current for generating the electromagnetic induction heat flows in the lengthwise direction of the opening portion of the stencil.
According to the above aspects of the present invention, the stencil itself is heated by induction heating, so that the temperature of the printing paste portion retained by the stencil (the portion of the printing paste that comes into contact with the inner wall surface of the through hole of the stencil and the portion in the vicinity of the portion) is increased more than in the inner portion, resulting in a reduced viscosity. As a result, the adhesive force of the printing paste between the stencil and the printing paste is reduced, as a consequence of which a resistance force when the printing paste is easily separated from the stencil becomes small to allow the stencil separation operation to be satisfactorily achieved. Therefore, no printing paste is left on the stencil side, so that no blur of printing is caused in the next printing stage and a specified amount of printing paste is supplied, that is, the printing paste is supplied in a specified shape to a specified position on the object on which a print is to be formed, thereby allowing a printing paste layer to be formed by printing. According to the above aspects of the present invention, the resistance of the printing paste in the inner wall surface portion of the through hole of the stencil becomes small. Therefore, a satisfactory print result can be obtained even when the stencil separation velocity is set higher (for example, not smaller than 1 mm/s and not greater than 3 mm/s) than the conventional stencil separation velocity (for example, not smaller than 0.1 mm/s and smaller than 1 mm/s) or without velocity control.
According to the above induction heating, the stencil itself generates heat, and therefore, the discharge of heat of the stencil can be immediately performed after the stop of the induction heating. Therefore, the portion other than the stencil is not heated, exerting no bad influence on the next printing operation, the devices around the stencil and so on. In contrast to this, according to the method of heating the stencil by externally radiating heat as observed in the case of hot air, radiation heating (infrared heating), or conduction heating, the members and air around the stencil are heated and the members and air around the heating unit, which also generates heat, are disadvantageously heated. Therefore, bad influence is sometimes exerted on the next printing operation, the devices around the stencil and so on. According to the method of transmitting heat from the heating unit to the stencil, the heat is conducted not only to the stencil but also to the heating unit and the members and air around the stencil, resulting in the drawback that heating efficiency is bad.
When performing induction heating in a noncontact manner without bringing the induction heating unit in contact with the stencil, the induction heating unit is not brought in contact with the printing paste on the surface of the stencil, and therefore, the induction heating unit is not smeared by the printing paste. In the case where an electronic component exists on the lower surface of the object on which a print is to be formed, the noncontact method can prevent the exertion of bad influence on the electronic component during the induction heating because of an increased distance from the electronic component.